Dental composite cements contain a polymerizable organic matrix and one or more fillers, which are usually surface-modified with a polymerizable adhesion promoter. Depending on the type of fillers, the monomer matrix and the application, the fill level can vary between approx. 20 and 80 wt.-%
As a rule, the polymerizable organic matrix contains a mixture of monomers, initiator components, stabilizers and pigments. Mixtures of dimethacrylates are usually used as resins. Examples of this are the highly viscous dimethacrylates 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (bis-GMA) and 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (UDMA) or the dimethacrylates of lower viscosity used as diluting monomers, such as e.g. bismethacryloyloxymethyltricyclo[5.2.1.]decane (TCDMA), decanediol-1,10-dimethacrylate (D3MA) and triethylene glycol dimethacrylate (TEGDMA).
In the radical polymerization of dental composite cements based on cross-linking dimethacrylates, a 3-dimensional polymer network already forms within a few seconds at the so-called gel point and thus a high mechanical strength develops, with the result that a simple, non-destructive debonding of the substrates or parts bonded using these cements, such as e.g. the bonding of a ceramic crown to a tooth stump, is possible only with difficulty or not at all. The early formation of a 3-dimensional network additionally results in a relatively high polymerization shrinkage stress (PCS) because the polymerization shrinkage can no longer be compensated for by a viscous flow of the monomers.
Adhesive bonds that can be debonded again are increasingly important in various technological fields. Examples are the detachment of components within the framework of automated manufacturing processes, the repair of complex components with adhesively bonded partial components or the simplification of the separation of materials when recycling such components at the end of the product's life. The debonding of adhesive bonds can be achieved on demand for example by significantly reducing the strength of the adhesive bond layer by heating.
Thus DE 198 32 629 A1 describes an adhesive system for forming reversible adhesive bonds based on polyurethanes, polyureas or epoxy resins, in which an additional component can be activated by introducing energy such that the adhesive components degrade. For example, organic bases or acids which degrade the adhesive resin can be released from blocked precursors by introducing heat or radiation energy.
WO 2010/128042 A1 describes industrial adhesive compositions for debondable adhesive bonds for aircraft or motor vehicle construction which consist of a customary adhesive matrix and a particulate expansion material such as e.g. azodicarbonamide. The components are debonded by heating the adhesive bond at least to the expansion temperature of the expansion material.
In dentistry, the debonding of adhesive bonds is important among other things in orthodontics, where brackets which are adhesively bonded to the tooth surface to correct malocclusions must be removed again without damaging the tooth enamel after successful correction. Moreover, in the case of repair or complete replacement of high-strength ceramic restorations or crowns which can be removed mechanically only laboriously, cement bonds that can be easily softened or separated would be advantageous.
In connection with orthodontic applications, US 2007/0142498 A1 describes dental compositions which contain thermally controllable additives such as e.g. thermoplastic polymers.
US 2007/0142497 A1 discloses dental compositions based on dimethacrylates with acid-labile tertiary carbonate groups and photoacids such as e.g. triarylsulfonium salts. These compositions can be photochemically cured using light in the visible range with suitable initiators such as, for instance, the bisacylphosphine oxide Irgacure 819 (photobonding) and softened again by irradiation with UV light at increased temperature (photothermal debonding).
WO 2013/034777 A2 and WO 2013/034778 A2 disclose dental materials which contain thermolabile or photolabile polymerizable compounds. The materials can be debonded again from the substrate by introducing heat or by irradiation with light in the UV or visible wavelength range.